1. Field of the Invention
The present invention relates to a method of driving optical output media in an optical writing apparatus, said apparatus being used for digital printers and the like.
2. Description of the Prior Art
As a solid-state scanning optical writing apparatus, an array type using optical output media such as LEDs, PLZT, fluorescers and liquid crystal, etc. aligned in one direction has conventionally been known.
For example, an optical shutter using PLZT as an example of the optical output medium is provided as a ceramic wafer comprising lead, lanthanum, zircon and titanium that are sintered, and is cut out or arranged so as to have a pixel pitch for the necessary printing density and resolution. The PLZT optical shutter transmits or does not transmit light according to the polarization direction of the light between when a voltage is applied and when no voltage is applied. Therefore, the PLZT optical shutter transmits or intercepts light from a light source according to whether a voltage is applied or not, and outputs the transmitted light. For this reason, no special light source is necessary. In addition, the PLZT optical shutter has a responsiveness of as high as in nanoseconds, and easily handles an increase in writing speed.
The inventor has developed a solid-state scanning optical writing apparatus using PLZT optical shutters as optical output media, that is, an A3-ready sliver halide printer employing a print head. In such a printer, in addition to the formation of A3-size images, the formation of images of a small size such as the L size in photography is naturally performed.
When a large-size image is printed after a small-size image is printed on photographic paper, there are occasions when there is a difference in output density on the same photographic paper between the part of the large-size image coinciding with the small-size image printed therebefore and the part of the large-size image not coinciding with the small-size image.
As a result of various experiments and examinations conducted by the inventor, it was found that the output density difference is caused due to an electro-optic effect, that is, an optical hysteresis and an electric field hysteresis of the PLZT optical shutter. FIG. 17 shows results of a measurement of the difference in quantity of the transmitted light with respect to the drive voltage between before and after the hysteresis due to the electric field. FIG. 18 shows results of a measurement of the difference in quantity of the transmitted light with respect to the drive voltage between before and after the hysteresis due to light. In either case, it is apparent that the drive voltage is changed by the hysteresis. FIG. 19 shows that the drive voltage varies with time due to the electric field hysteresis.
Thus, in the PLZT, when the hysteresis is caused, a value called a halfwave voltage where the quantity of the transmitted light is largest is varied, and this results in image writing non-uniformness and image output density non-uniformness. When the applied voltage is high or when an electric field is steadily applied like in a DC state, as shown in FIG. 19, the hysteresis quickly progresses.